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Guide to measuring PDN on a budget

Posted: 20 Jun 2014 ?? ?Print Version ?Bookmark and Share

Keywords:vector network analyser? S-parameter? power distribution network? PDN? VNA?

As the use of microcontrollers, CPUs and FPGAs is ever increasing, more engineers are facing challenges with their power distribution network (PDN) design. Many engineers are just starting to perform PDN measurement. Typically, these measurements are performed using a vector network analyser (VNA). A 1-port measurement can be used for impedance levels above a few hundred m. A 2-port shunt-thru measurement is used for applications as low as 1m and in some cases even lower. Both of these measurements are based on S-parameters, allowing the cable to match the instrument impedance all the way to the point of measurement. (For background on s-parameters, head to the S-parameter collection).

There are many good analysers available for measuring PDN
High performance VNAs are available from all of the major test equipment manufacturers, including Agilent, Tektronix, Teledyne Lecroy, and Rohde & Schwarz. Most product lines include offerings of S-parameter measurements up to 40GHz or higher.

Many of these are above our price range
While these are all excellent instruments, and I use them myself, we dont all have a budget that can support such an expense. There are other options available. If you only need an instrument for a short time, you might consider renting one from one of the many test equipment rental companies. Depending on your measurement requirements, you might be able to make use of some lower cost instruments, two of which we will discuss in this article.

What frequency range do you need to measure?
A recent poll on our Power Integrity for Distributed Systems LinkedIn group indicated that 22% of the voters only measure PDN to 40MHz, while 33% measure to 200MHz and 44% measure to 4GHz.

Figure 1: 22% of respondents only need to measure PDN impedance up to 40MHz or less while all respondents measurements can be fulfilled with a 4GHz capability.

None of the respondents currently measure PDN above 4GHz. As this unscientific poll shows, we arent all designing the highest speed circuits. That does not mean that we wont have PDN problems, but it could limit the maximum measurement frequency, and measurement bandwidth is expensive. A general rule of thumb for the maximum measurement frequency is based on the highest edge speed of the loading circuits. The 3dB corner frequency is approximately

Where T is the rise or fall time of the load current. It is good practice to measure up to at least twice this frequency. At the lower limit, we have to be sure to include the lowest possible regulator bandwidth and measure at least an octave below that. Many switching regulators and LDOs have measured bandwidths of a few kHz minimum. Its safe to say that measurement should start from about 1kHz.

What instrument characteristics are important?
Assuming the VNA has typical SOLT (short, open, load, and thru) calibration capability, measuring PDN requires sufficient dynamic range for the measurement. The 2-port impedance measurement has a resulting S21 magnitude of

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